1. Field of the Invention
The present invention relates to accessories used in conjunction with spectrophotometers and spectrometers, more particularly with Fourier Transform Infrared (FTIR) spectrometers. These accessories use infrared-transmitting optical fibers to enable spectral analysis of samples remote from the body of the FTIR spectrometer and, in particular, are adaptable for use in monitoring chemical reactions and chemical manufacturing processes which may be proceeding at elevated temperatures and/or pressures.
2. Prior Art
The use of optical fibers and, in particular, optical fiber cables, to enable collection of FTIR spectra directly from chemical reactors or other sites without the need to remove samples and transfer them to a compartment inside the spectrometer is well known. For example, a fiber-optic probe has been used to obtain spectra from an electrochemical reactor (Shaw and Geiger, Organometallics, 1996, 15; 13-15) in which air-sensitive species and transient intermediates were identified in situ, and kinetic data were obtained in a convenient manner using a standard electrochemical reactor, without perturbing the course of the reaction or admitting air to the reactor. When this method is used in the mid-IR region of the spectrum (approximately 700-5000 cm.sup.-1), it is often necessary to use optical fibers made of materials, typically chalcogenide or fluoride glasses, that are liable to undergo phase transformations such as melting or crystallization at comparatively low temperatures. Means exist to provide some protection for the optical fibers (which typically do not directly contact the medium under examination) by, for example, adding a cooling jacket which circulates water around the outer casing of the optical fiber cable (see copending patent application Ser. No. 08/380,078). This means is effective for cases where the temperature is elevated but still comparatively modest (for example, up to approx. 100 deg. C.), and in circumstances where there is no practical objection, such as a safety hazard, to the use of water close to the reactor or other vessel. However, in cases where temperatures are considerably higher--up to 400 deg. C. for example--the degree of cooling that can be provided by the water jacket is inadequate, and it may be impractical or undesirable to use water in close proximity to the reactor or vessel under examination.
A particular situation where remote spectroscopic monitoring in real time can be highly advantageous is in the field of plastics or polymer extrusion. Plastics are extruded in a fluid, but usually somewhat viscous, state at temperatures ranging up to well above 300 deg. C. Chemical reactions, including polymerization reactions and other changes, can take place during extrusion inside the extruder. Other parameters, such as the volumetric distribution of particulates such as fillers, colorants, antioxidants, stabilizers, etc. or the homogeneity of polymer blends or alloys, may also be subject to change or variation under the conditions of temperature, pressure, flow, etc. that prevail inside the extruder. Because of the importance of monitoring all of the various quality- and composition-related parameters in extruded materials, considerable resources are often expended to provide quality control laboratory facilities; however, the process feedback from such laboratories is often slow, making process control based on laboratory results difficult and compromising the real-time nature of the results. A better approach is to devise an on-line or in-line method for monitoring the material in the extruder.
Several attempts have been made to use near infrared (NIR) spectroscopy for monitoring of polymer processes in the extruder. For example, an anonymous Research Disclosure (RD 29959, March 1989) describes the use of a NIR probe based on a sapphire or quartz rod to obtain real-time, in-line NIR data from polymer melts; the probe can be designed to fit into a standard well, such as a Dynisco Pressure Well, in an extruder. Others, including McPeters and Williams (Process Control and Quality, 1992, 3, 75-83) and Khettry and Hansen (Polymer Engineering and Science, 1996, 36, 1232-1243) have used fiber-optic cables based on silica fibers to couple NIR spectrometers to probes for in-line monitoring of polymer melts in extruders. All of these NIR methods have a number of serious, inherent disadvantages. Because of the low extinction coefficients (i.e. low molecular absorbance) exhibited by NIR radiation, relatively long path lengths are required; this means that transmission or transflectance techniques have to be used, leading to the use of complex sampling elements that interfere with the flow inside the extruder; this is in contrast to the convenience of evanescent wave techniques such as attenuated total reflectance (ATR), which can be used successfully in the mid-IR region of the spectrum where higher extinction coefficients prevail. Another problem arises from the nature of NIR spectra, which are characterized by weak bands which tend to be broad and overlapping, and by scattering of the NIR signal in multiphase systems such as polymer blends or polymers containing fillers. As a result, spectral interpretation in the NIR is almost always based on mathematical analysis by the partial least squares (PLS) method, which requires the generation of extensive calibration sets of spectra based on predetermined mixtures of standard compounds. The mid-IR region of the spectrum exhibits higher extinction coefficients and thus shorter path lengths can be used to obtain usable spectra; this enables the use of the ATR method rather than transmission or reflectance. Furthermore, mid-IR spectra comprise sharper better separated peaks than those in the NIR, obviating the need for PLS methods of interpretation, with the accompanying laborious calibration, in most cases. However, previous attempts to use mid-IR spectroscopy as a tool for monitoring chemical events inside an extruder (see, for example, H. L. McPeters, Anal Chim. Acta, 238 (1990), 83) have utilized complicated methods of sample handling by creating a side stream of material from the extruder into a short path-length transmission sample cell. While this method has the advantages over NIR methods that the spectra comprise strong peaks and contain a wealth of chemical information, and that they are in most cases able to be interpreted without the use of mathematical methods such as PLS, the removal of a side stream requires mechanical adaptation of the extruder, perturbs the flow in the extruder and compromises the real-time nature of the experiment, which is best described as on-line rather than in-line. The best solution to the above problems would be to use a fiber-optic probe operating in the mid-IR, and such devices are well known (see, for example, U.S. Pat. No. 5,170,056). This would permit true in-line monitoring of the polymer melt without the need for PLS interpretation. However, since fiber-optics capable of transmitting in the mid-IR are frequently made from materials such as heavy metal fluoride glasses or chalcogenide glasses which are subject to optical deterioration at temperatures close to or above 100 deg. C., their use in proximity to a plastics extruder or any other apparatus heated above about 200 deg C. has been a practical impossibility until the advent of the present invention.